You can resupply supply trucks only at a FOB. However, you can give more than one order to your units by pressing shift. Hold shift, then order your supply trucks to fast move back to your FOB, right click the FOB (to resupply), fast move back to the front, release shift.

Looks like you already got help but yes you can resupply supply trucks even in the first mission, if you do the secondary objective to destroy the HQ unit across the bridge, which is near an enemy Forward Operating Base which you can capture and send your supply trucks to. However you may not want to go for it until you play through some missions and spend points to get some tougher units (you pretty much go through the Iron Curtain and there's a few enemy units that recon has trouble seeing (because of the Iron Curtain in the way) and plus my recon was destroyed because it had to get right on top of them to see them.

multi-moving your supply trucks from the front to the FOB and back is a good idea however make sure you adjust the trucks' destination if your front advances. Just as in real life, you don't want to outrun your logistics. Also it can be good to invest in a helo logistics unit because they don't have to follow roads obviously, they can fly straight to the FOB and back.

I've heard it said that you can create a chain of supply trucks by putting them at the border of each other's supply borders. I did actually see this effect when I put a supply truck towards the edge of an FOB and it bulged the supply limit outward.